1. Field of the Invention
This invention relates to thermostatic controls for building heating and/or cooling systems and more particularly to such systems which adaptively modify their operation as a function of changes in the rate of temperature exchange between the building and its environment resulting from changed environmental conditions.
2. Prior Art
Thermostat control systems for heating furnaces and/or air cooling systems (hereinafter collectively referred to as "furnaces") of the type employed in residences and many commercial and industrial buildings generally include means for manually entering a desired temperature set point, means for measuring the actual temperature within the building, and means for switching the furnace on or off as a function of differences between the set point temperature and the actual temperature. The ideal or desired temperature within a building will usually vary over a repetitive time cycle such as day or week. For example, in a residence that is heated by a furnace it is usually desirable that the night-time temperatures, when the occupants are normally sleeping, be lower than the day-time temperatures. Thermostats therefore have been devised in which the 24 hour day may be divided into two periods and different desired temperature set points may be programmed for each of the periods. My copending patent application Ser. No. 778,376 entitld "Electronic Thermostat" discloses a system including a digital memory for storing a lengthy schedule of desired temperatures for times within a repetitive cycle. The memory is interrogated by a digital clock to output a signal representative of the desired temperature set point at any time and this temperature is compared to the measured building temperature to control the operation of the furnace. This system allows the provision of relatively complicated time-temperature programs to accommodate to the practices of the occupants of the building.
In programming these systems the operator must estimate the rate of temperature change which occurs when the furance is switched on in order to meet important time temperature points. For example, suppose the residents of a house want to maintain a 60.degree. F. night-time temperature but want to have the temperature increase to 70.degree. when they arise at 7 a.m. In relatively warm weather, i.e., with an external temperature of 50.degree. or so and no appreciable wind, the rate of heat transfer between the building and its environment will be relatively low and it may be possible to make the 10.degree. temperature excursion from 60.degree.-70.degree. by starting up the furnace 1/2 hour before the temperature change must be completed, or at 6:30 am. In colder weather and with more wind the rate of heat transfer between the building and its environment will be increased and the furnace will have to be switched on at earlier times in order to meet the 7 am. schedule. On a windy day with a 10.degree. F. external temperature it may be necessary to turn on the furnace of 5 am. in order to obtain the desired 10.degree. internal temperature rise by 7 am. While the operator might adjust the programmed turn-on time seasonally or even daily to compensate for variations in the weather, more likely a single turn-on time will be programmed to accommodate the worst possible condition, and accordingly during warmer temperatures the 70.degree. temperature will be attained before 7 am. and there will be some resulting waste of fuel. Since the lowest exterior temperatures usually occur during the early morning hours the resultant fuel waste can represent a substantial fraction of total fuel use. Additionally, the excessively warm early morning temperature may be uncomfortable for the sleeping residents.
A similar situation occurs during seasons when a cooling or air conditioning system is in use and a residence is unoccupied during the mid-day hours, but the residents desire to have a relatively cool temperature during the early evening hours. With a conventional single setting thermostat a temperature slightly higher than the desired evening temperature may be programmed before the residents leave and when they return in late afternoon the set point may be lowered to the desired temperature. With a multiple set point system the thermostat may be programmed to a relatively high set point during the hours that the building is unoccupied and the lower desired temperature may be programmed for a time beginning before the residents return. Again, the initiation of this precooling period to attain the desired occupancy temperature will typically be based on the programmer's estimate of the time required to lower the temperature from the afternoon setting to the occupancy setting under the worst circumstances. During periods when the temperature difference between the interior and the exterior is relatively minor, and accordingly there is a low rate of heat exchange between the interior and the exterior, this precooling period will prove to be excessively long and the building will be maintained at a cooler than necessary temperature for some period of time during the late afternoon, the hottest time of the day.